1. Field of the Invention
This invention relates to an improvement in a burner for burning powdered coal, and more particularly, to a burner suitable for directly heating an object to be heated in a heating vessel.
2. State of the Art
When, for instance, iron scrap is melted and refined to produce steel, it is preheated in a heating vessel before melting. The methods of preheating scrap are divided into two types: one is a low temperature type in which scrap is heated up to an average scrap temperature of 300.degree. to 400.degree. C., and the other is a high temperature type in which scrap is heated up to a temperature not lower than 600.degree. C. A steel vessel is good enough for the former type, but the latter type requires a vessel with a refractory lining. Scrap preheating by a powdered coal burner is mainly of this high temperature type.
The characteristics required of a powdered coal burner for this kind of purpose are as follows:
The length of the flame should be short. PA1 The temperature of the flame should be appropriately low. PA1 Ignition and combustion should be stable. PA1 The amount of NOx generated in the burner should be as small as possible. PA1 Oxidation loss of heated scrap should be small. PA1 The structure of the burner should be simple and the maintenance thereof is easy.
There are two different kinds of burners for powdered coal: one is a burner using pure oxygen, and the other is a burner using air. It was found that, since the flame temperature of a pure oxygen burner is high and the oxidizing power of the atmosphere is strong, oxidation of the scrap and unevenness of temperature of scrap is inevitable when such a burner is used for preheating scrap. Therefore, basically an air burner or an oxygen-enriched air burner should be used.
Even in an air burner, because the flame temperature is as high as 1800.degree. C., heat loss to the refractory which forms the combustion space is high. In order to reduce this heat loss, it is necessary to make the combustion space smaller, and hence, to shorten the flame length to the utmost. As a way of shortening the flame length, it is tried to use small-sized burners with reduced fuel input and to install many of these small-sized burners in one vessel. This way of shortening the flame length has limited effect, and it is desired to provide a burner which burns with a short flame regardless of the size of burner.